This invention relates to a process for internally strengthening paper or board products during their manufacture and to the resulting products having enhanced properties. The papermaking industry as well as other industries have long sought methods for enhancing the strength of products formed from fibrous materials such as, for example, paper and board products formed of cellulose fiber or pulp as a constituent. The problems and limitations presented by inadequate dry strength have been particularly acute in the numerous industries where recycled or mechanically ground furnish is utilized in whole or part. In the papermaking industry for example, recycled cellulose fiber is typically used in the manufacture of newsprint and lightweight coated papers. These recycled fibers, however, are of a generally shorter length than chemically-pulped fibers which in turn provides paper having relatively poor dry-strength properties in comparison to paper manufactured from virgin, chemically pulped fiber. The use of virgin, chemically pulped fiber for all paper and board production, however, is extremely wasteful in terms of natural resource utilization as well as cost prohibitive in most instances and applications.
Various methods have been suggested in the past for improving the dry-strength and related properties of a sheet formed from fibrous materials such as paper or board materials formed of cellulose fiber. One alternative for improving the dry-strength properties of paper products, for example, involves the surface sizing of the sheet at a size press or calendar stack after its formation. While some of the critical properties of the product may be improved through sizing the surface of the sheet, most papermaking machines, for example, including board, coated publishing and newsprint machines, are not equipped with a size press. Moreover, only the properties of the surface of the sheet are appreciably improved through surface sizing. Surface sizing therefore is either not available to a large segment of the industry or is inadequate for purposes of improving the strength of the product throughout the sheet. The latter factor is especially significant since paper breaks during printing, for example, are obviously disruptive to production and extremely costly.
A preferred alternative to surface sizing of a sheet is to increase the strength of the product through the addition of chemical additives directly to the fiber furnish prior to forming the sheet. Common additives at the wet-end of a paper machine, for example, include cationic starch or melamine resins. The problem presented by known wet-end additives used in the papermaking industry, however, is their relatively low degree of retention on the cellulose fiber during the initial formation of the sheet at the wet-end of the paper machine. In most applications, significant portions of the wet-end additives accompany the white water fraction as it drains through the wire due to high dilution and the extreme hydrodynamic forces created at the slice of a fourdrinier machine, for example. Alternatively, a significant portion of the additive may be lost in solution during the dwell time between its addition to the stock and the subsequent formation of the sheet on the machine at prevailing operating temperatures. Accordingly, the potential benefits achievable through the use of known methods for internally strengthening fiber products have seldom been realized in practice. And, when the cost of the chemical additives is additionally considered, any marginal benefits actually achieved have been largely disappointing.
A previously known and particularly desirable surface sizing agent applied in the paper industry is polyvinyl alcohol. The use of polyvinyl alcohol as a surface sizing agent or adhesive is described, for example, in U.S. Pat. Nos. 2,330,314 to Schwartz; 3,183,137 to Harmon et al.; 3,276,359 to Worthen et al.; and 3,878,038 to Opderbeck et al. Other patents have additionally described the use of polyvinyl alcohol as a surface sizing agent following the use of different compositions as wet-end additives, such as melamine formaldehyde resin, as described, for example, in U.S. Pat. No. 3,773,513 to MacClaren. In addition, U.S. Pat. No. 4,372,814 to Johnstone et al., describes the use of fully hydrolyzed polyvinyl alcohol as a "binder" for a distinct group of wet-end additives and again, thereafter, as a surface sizing agent.
U.S. Pat. No. 2,402,469, Toland et al., describes the use of polyvinyl alcohol as a wet-end additive to improve the wet-strength as opposed to dry-strength properties of the sheet. The addition level proposed in the Toland patent, however, is approximately ten percent on an oven-dried weight basis of the pulp, apparently reflecting extremely low-retention at the wet-end even at the relatively low paper machine operating speeds which prevailed at that time. In addition, the polyvinyl alcohol product described in Toland et al. is soluble in water at 130 degrees Fahrenheit. Since many paper machine chests are maintained at prevailing temperatures of 130 degrees Fahrenheit, or higher, the process described in the Toland et al. patent would therefore be ineffectual in most, if not all, papermaking applications.
In a 1973 publication by John Wiley and Sons on the subject of polyvinyl alcohol, Chapter 12 is devoted to discussions of the use of this product in paper manufacturing. This chapter was authored by two employees of Nippon Gohsei Co., Ltd. of Osaka, Japan. In Section 12.4, the subject of "internally sizing" paper with polyvinyl alcohol is addressed and references the above-noted Toland, et al. patent and additionally Japanese Patent No. 12,608 relating to layered board and assigned to Nippon Gohsei. The publication describes the desirable properties of a polyvinyl alcohol product which purportedly can be used as a wet-end additive and identifies a particular grade sold by Nippon Gohsei, "Gohsenol P-250," as suitable for direct addition to beater size. The Gohsenol P-250 product is described in the publication as 98-99 mole percent hydrolyzed and as having a dissolving temperature of 67-70.degree. C.
In a 1982 technical paper presented during the 1982 TAPPI Papermakers Conference, Dr. David Zunker of E.I. duPont de Nemours & Company, Inc. describes the significant problem in achieving any retention of polyvinyl alcohol at the wet-end of a paper machine. In that paper, the use of mixtures of polyvinyl alcohol and cationic trimethylolmelamine as a binder is proposed as a solution to the retention problem. The use of "TMM" as proposed by Dr. Zunker, or alternatively the use of cationic starch as a retention aid for polyvinyl alcohol, has not been successful, however, because the negatively-charged anionic white water quickly neutralizes the positive, cationic charges of the starch or TMM after the paper machine reaches equilibrium in its white water system. In addition, TMM is a known enhancer of wet-strength properties which presents distinct problems in repulping any fully dried broke for reuse as furnish.
In view of the foregoing, it is a primary object of the present invention to provide a method for internally strengthening products formed from fibrous materials, and especially paper and board products, by successfully incorporating polyvinyl alcohol of defined properties into the formed sheet prior to drying. It is an additional object of this invention to provide a method which results in surprisingly high retention of the polyvinyl alcohol on the pulp fiber even at relatively high operating temperatures in order to obtain the maximum benefit from the use of the additive, including enhanced strength properties at economically feasible levels of addition.